1) Obliques. You have both internal and external obliques and they are really big, potentially strong muscles that need specific exercises to target them.
Generally speaking, your body's largest muscle groups, such as those in your back, chest, and legs, are likely to respond most quickly to strength training. Increased strength and mass come from forcing the muscles to work, and it is easiest for you to overload these muscle groups with heavy weights.
In general, muscles that have a higher proportion of slow twitch fibers will be harder to grow than those that don't. Most people have a very high proportion of slow twitch fibers in their calves, so they're going to be particularly difficult to grow.
This theory is supported by the finding that the pecs are the slowest muscle group to recover following training (11). I think 2x/week for chest training is plenty as the pecs will need at least a few days to recover between sessions.
Answer and Explanation: (a) The muscle type that is slow to contract is smooth muscle. It is known to produce slow contractions because the smooth muscle has calcium channels which open much more slowly that other muscles.
Glutes and Hips
The glutes and hips are some of the most common weak muscles.
Glute Muscles
Perhaps the most neglected muscle group in the body, the Glutes are also one of the most important muscle groups for proper biomechanics and optimal sports performance. They're also connected to your spine, so weak Glutes muscles can lead to back pain and injury.
As the refractory period of cardiac muscle is high, it does not get fatigued. Therefore, cardiac muscles work all the time throughout life without undergoing fatigue.
It seems to easiest muscles to grow are upper body muscles such as the arms, shoulders, back and chest in men. And in women, the easiest muscles to grow are in the lower body such as the legs and glutes. However, there is more to it than this and it's not that simple, genetics are only one factor.
Typically, muscle mass and strength increase steadily from birth and reach their peak at around 30 to 35 years of age. After that, muscle power and performance decline slowly and linearly at first, and then faster after age 65 for women and 70 for men.
Late teens and early twenties are the perfect age to start bodybuilding. Puberty and bodybuilding are closely related because this is the fastest time for muscle growth. Between, 17-25, you will experience testosterone driven growth burst in your muscles.
Progressive overload and overload in and of itself is pretty difficult to achieve with the biceps. You need to do something dramatically different in your arm workouts to stimulate those muscles, and that is to vary the way in which you're performing your biceps curls!
Fast-twitch muscle fibers provide bigger and more powerful forces, but for shorter durations and fatigue quickly. They are more anaerobic with less blood supply, hence they are sometimes referred to as white fibers or type II.
Remember, weighted squats trigger a flood of growth hormone into the body because of the sheer amount of muscles being activated. It's one of the reasons your legs are likely to grow faster than your arms.
Auricular Muscles
Aside from those special few that are able to wiggle their ears as a party trick, most people are not capable of manipulating these muscles at all. Evolution has deemed the auricular muscles unnecessary.
Cardiac muscles are built to pump and transport blood from the heart to the rest of the body. The refractory period of cardiac muscles helps them to avoid fatigue and work throughout their lifetime.
Final Answer: Hence, The muscles which never fatigue are the Cardiac muscles.
Your pushing days should focus on the muscles of your chest, shoulders and triceps; pulling days on your back and biceps; and lower days on movements such as squats, deadlifts and lunges for those legs.
However, it's incredibly difficult to grow your lats, and if you're trying to do so, you might be frustrated due to a lack of progress.